Statistics of a combined paleomagnetic data set from six studies of Mississippi Valley–type deposits and their surrounding host rocks show that the characteristic remanent magnetization of the host rocks predates the magnetization of the mineralization, providing a positive contact test confirming that the ore magnetization is primary. Either multiple fluids or dramatic changes to a single fluid through time are required to account for the separate magnetization ages observed in the host rocks and ores. The dispersion of the characteristic remanent magnetization directions provides an estimator for the duration of the mineralizing process, suggesting a mineralizing event of about 4 m.y. duration.

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